The invention relates generally to determining equivalence ratio of a gas-fuel mixture and more particularly to a system and method of measuring an equivalence ratio of the gas-fuel mixture in a gas turbine engine in real time.
In order to reduce NOx emissions and increase lifetime for gas turbine engines for power and propulsion systems, a lean premixed combustion is widely preferred. In lean flames, the dominant NOx formation mechanisms depend on the local flame temperature. The gas turbine engines that operate at lean fuel/air equivalence ratios significantly reduce NOx production due to lower flame temperatures. In addition, lower flame temperatures reduce maintenance requirements for gas turbine components such as combustion liner. Thus, equivalence ratio is a key parameter for operations of a gas turbine engine. However, the lean premixed combustion is susceptible to thermoacoustic instabilities and lean blowout, thereby, reducing efficiency and increasing emissions. This further leads to hardware damage and causes safety hazards.
Furthermore, thermoacoustic instability is a self-sustained combustion oscillation near the acoustic frequency of the combustion chamber, which is the result of the closed loop coupling of unsteady heat release to pressure oscillations. Intensive experimental and theoretical work has been performed to understand the driving mechanisms of thermoacoustic instabilities, and to suppress these instabilities in laboratory-scale and full-scale combustors. It is well understood that heat release fluctuations can produce pressure oscillations; however, the mechanisms whereby pressure oscillations result in heat release fluctuations are not well known. Equivalence ratio fluctuation is considered to be one of the most important driving mechanisms for thermoacoustic instabilities in fuel-lean gas turbine combustion systems. Because of the complex physical and chemical interactions involved in thermoacoustic oscillations, it is difficult to predict this unstable combustion behavior. Therefore, measurement of the equivalence ratio fluctuation during unstable combustion is of great importance for monitoring thermoacoustic instabilities in the gas turbine engines. In addition, measured flame transfer function between the equivalence ratio fluctuation and the heat release fluctuation can be used as direct input to the analytical model to predict combustion instabilities.
Equivalence ratio has been measured using infrared (IR) methane absorption of the 3.39 μm wavelength output of a He—Ne laser to study its effect on heat release during premixed unstable combustion (lab scale). Local fuel-to-air ratio was also measured by laser absorption at the same wavelength to study the effect of mixing on NOx emissions in premixed burner. The same IR laser absorption technique has been also used to measure fuel concentration in pulse detonation engines and internal combustion engines. However, He—Ne lasers are sensitive to ambient conditions and simultaneously emit diffuse radiation and coherent light at multiple discrete wavelengths. In addition, the absorption at 3.39 μm wavelength is the carbon-hydrogen (CH) asymmetric stretch bond common to all hydrocarbon fuels, while different hydrocarbons have different absorption coefficients. Therefore, the sensor needs to be calibrated for each fuel mixture encountered during operation. Thus, the IR absorption method has limitations for practical application in gas turbine engines.
Moreover, the current gas turbine operations rely on overall flow splits to estimate the average flame temperature, and adjust fuel/air ratio for optimal operation in terms of combustion stability and emissions like CO and NOx. However, the capability of this method is limited due to uncertain nozzle-to-nozzle and can-to-can flow variations.
Accordingly, there is an ongoing need for accurately and rapidly measuring an equivalence ratio of the gas-fuel mixture in real time in practical gas turbine engines.